Recently, the photovoltaic cell or solar cell market is rapidly growing thanks to worldwide investments in the alternative energy industry and green growth policy, and production of solar cell modules is expected to grow at an annual rate of 50% or higher. In particular, according to NanoMarkets (a photovoltaic industry analyst, in Glen Allen, Va.), solar cell modules are predicted to produce up to 34.7 Gigawatts (GW) by 2015 up from 6 GW in 2008.
However, because of structural problems in solar cells using crystalline silicon which lead to a high price of solar cell modules since their manufacturing process is very complicated and the productivity is very low because of batch production, thin film solar cells using amorphous silicon or other products formed by depositing silicon using stainless-steel or polyimide, which is very flexible and is thin and light, as a substrate have been developed lately. However, though this kind of products has an advantage of being relatively light in weight and of reducing the production cost thereof, the solar cells of this type have an efficiency of about 6% which is much lower than that of the crystalline silicon solar cells which is about 7 to 20% and have a drawback of a shorter service life.
Because of the problems described above, though attempts have been made to develop solar cells harnessing photovoltaic effect of organic materials instead of using silicon, the solar cells using such organic materials have a disadvantage of a low energy conversion efficiency and most of all have an issue in durability. Michael Grätzel who is a professor at the École Polytechnique Fédérale de Lausanne in Switzerland proposed a new type of solar cell, so called a dye-sensitized solar cell consisting of photosensitive dye particles and titanium dioxide of nanoparticles, and reported a result of a very high energy conversion efficiency of around 10% comparable to solar cells based on amorphous silicon series among conventional inorganic solar cells. It has been known that the above dye-sensitized solar cell has a very high possibility of commercialization because its production cost is just around 20% of that of silicon solar cells, and accordingly there have been a lot of research studies around the world for real applications.
Most importantly, low economic feasibility has been an issue because of a high production cost of silicon solar cells and a limitation of efficiency of a cell itself over the course of development of the solar cells described above. In addition, there is an urgent need for development of solar cell modules with easy accessibility as an alternative energy in daily lives. While the industry has practically focused mainly on the development in the field of installing silicon-based solar cells so far, future development of solar cells is expected to pioneer new applications suitable for respective properties of dye-sensitized solar cells, organic solar cells, and thin film solar cells, and there is an increased need for a new technology at the same time. In particular, whereas silicon solar cells can generate electric power only with a very large amount of light, since dye-sensitized solar cells can produce electric power even with a small amount of direct rays of the sun, the dye-sensitized solar cells can generate electric power more efficiently than the silicon solar cells as a building integrated photovoltaic system for generating electric power using building walls, windows and so on. Therefore, though silicon solar cells may further be developed as a large-scale power plant in the future, large portions of photovoltaic generation using buildings in daily lives are expected to be covered by dye-sensitized solar cells. Besides, since applicability of such solar cells is anticipated to expand to various electronics or small portable gadgets using indoor lightings, automobiles, and even to clothes because of environment-friendliness, transparency and coloring, and efficiency under a low amount of light, a lot of research studies in the industry are being carried out for commercial availability.
Such a dye-sensitized solar cell was first invented by professor Michael Grätzel of Switzerland based on the principle of photosynthesis of plants, and consists of a working electrode, a layer of inorganic oxide such as titanium dioxide having dye molecules adhered thereto, a liquid electrolyte, and a counter electrode. Photo-electric conversion occurs through photo-electric chemical reaction between electrodes, a brief explanation of such process is as follows.
First, a working electrode is composed of an oxide semiconductor of nanosize having a molecular dye adhered thereto that absorbs sunlight to emit electrons. When external light reaches the molecular dye, electrons in the dye are excited to an elevated level of energy and then are received by the oxide semiconductor to transfer outside. The electrons with an elevated level of energy consume their energy while flowing through the external circuit, and then arrive at a counter electrode. The dye in the working electrode which emitted electrons receives electrons back through the electrolyte, and such an oxidation-reduction process occurs continuously in the course of supplying energy through ion transfer within the electrolyte.
Therefore, electrolytes play a very important role to transfer electrons through ionization, and especially the contact area between the electrodes and the electrolyte determines the amount of electric power produced. In other words, since the wider the contact area is, the faster and the more reactions can occur and the more the amount of the dye can be adhered, nanoparticles are used as materials of the respective electrodes. If nanoparticles are used, the surface area of the material significantly increases for the same volume and accordingly the material can have a larger amount of dye adhered to the surface thereof, thereby increasing the speed of electrochemical reactions between the electrode and the electrolyte. In general, a semiconductor oxide electrode of titanium dioxide for forming the working electrode is coated with a thickness of 10˜20 μm of nanoparticles having a size in the order of 20˜50 nm, and the dye adheres to the surface thereof. In addition, the counter electrode is coated with a thin layer of platinum particles having a size less than 10 nm on the substrate thereof.
On the one hand, dye-sensitized solar cells using conventional liquid electrolytes have inherent problems in stability and durability such as degradation of properties because of leakage of the electrolyte and evaporation of solvent, thereby hindering it from commercialization. In particular, it is impossible to manufacture such dye-sensitized solar cells into ones with large area because of a process of injecting electrolytes, which in turn makes it hard to achieve a low production cost which is one of the main benefits of the dye-sensitized solar cells. In view of such fact, there is an urgent need for development of solid or semi-solid electrolytes to replace liquid electrolytes.
For the reasons described above, the industry has made efforts to replace liquid electrolytes with solid or semi-solid electrolytes. There has been research and development using organic polymers or inorganic hole transfer materials (HTM) for such solid or semi-solid electrolytes, and the main research target has been organic polymer electrolytes because of its advantages in commercialization. This is because such organic polymer electrolytes make it possible to transform the shape thereof during the manufacturing of solid dye-sensitized solar cells so as to provide flexibility, and to manufacture thin films using techniques such as spin coating, which also serves as one of the advantages. In addition, they can maintain stable performance under thermal stress or light soaking compared with the liquid electrolytes, and can contribute to improvement in long-term stability and provide an advantage of low production cost. Examples of commonly used polymers include PEO, poly(propylene oxide) (PPO), poly(ethylene imine) (PEI), poly(ethylene sulphide) (PES), poly(vinyl acetate) (PVAc), poly(ethylene succinate) (PESc) and so on, and there have been research studies on the polymers described above because it is known that ion movements within the polymer electrolytes occur in the amorphous region by means of segmentation movements of polymer chains. Among the above, a complex of poly(ethylene oxide) (PEO) and alkali metal salts is the most widely known polymer electrolyte.
A polymer electrolyte was first proposed by preparing a complex of poly(ethylene oxide) and alkali metal salts by Wright group in 1975, and thereafter started being applied to lithium batteries of polymer electrolytes and to electrochemistry by Armand et al. in 1978. The majority of the conventional polymer electrolytes that have been reported so far are based on PEO, and PEO has been the most reported as a representative material for polymer electrolytes in the field of fuel-cells ever since the electrical conductivity according to mixing with metal salts was disclosed. Based on such reports, PEO has become one of the research topics getting the most attention among polymer electrolytes for dye-sensitized solar cells. This is because PEO is applicable to solid dye-sensitized solar cells since it exhibits various properties depending on its molecular weight, has excellent chemical stability, and shows a higher mechanical strength compared with liquid electrolytes. In particular, it is known that PEO has a regular arrangement of a large amount of oxygen atoms and an ion transfer mechanism for transferring metal cations by way of a spiral structure formed by polymer chains. Besides, polymer electrolytes are preferably composed of metal salts with low lattice energy, for example alkali metals such as LiI, KI, NaI and the like and polymers having polar groups capable of dissociating the metal salts. Thus, the polymers need to contain lone pair electrons capable of giving electrons, such as ones from oxygen (O) or nitrogen (N), and the polar groups make a coordinate bond with metal cations, forming a complex of polymer-metal salts.
However, since PEO basically exhibits high crystallinity for a high molecular weight, it has an essential limitation of a high molecular weight in consideration of durability. Such crystallinity of PEO (about 80%) serves as a disadvantage for having low ionic conductivity (10-8 to 10-5 Scm-1) and diffusion coefficient at room temperature. In addition, depending upon the chain size of polymers, how much of the polymer electrolytes can infiltrate pores of the oxide layer of a nano-sized titanium dioxide is an important factor and it is difficult for PEO with a high molecular weight to infiltrate the oxide layer, which in turn not only reduces energy conversion efficiency and but also shows a limitation on manufacturing solar cells in practice. Therefore, though there have been research studies on various methods to lower crystallinity of electrolytes based on PEO, to increase ionic conductivity and diffusion coefficient, and to enhance energy conversion efficiency by improving interfacial contact, the results so far just remain insignificant.
A main reason for the above problems is that ionic conductivity is inherently slow in a solid phase. In order to overcome such a drawback, researches related to semi-solid or gel-type polymer electrolytes utilizing intermediate characteristics between liquid and solid have also been actively carried out. Korean Patent Application Publication No. 2003-65957 describes semi-solid polymer electrolytes by way of example, and asserts that the semi-solid polymer electrolytes exhibit a high ionic conductivity similar to that of liquid electrolytes at room temperature. However, the semi-solid polymer electrolytes have lower durability when compared with solid polymer electrolytes because they exhibit poor mechanical properties such as glass transition temperature (Tg). The process for manufacturing solar cells is difficult because of semi-solid characteristics, and it is hardly possible to completely prevent electrolyte from leaking because solvents are mixed therewith.
As described above, most of the polymer electrolytes are based on poly(ethylene oxide) PEO, and accordingly it is important to increase the amorphous region by lowering the crystallinity thereof. To this end, improvement of ionic conductivity, and decrease of crystallinity through addition of nanoparticles, crosslinking, blend, formation of copolymers or the like are the main research topics in polymer electrolytes, and it is possible to obtain an additional performance improvement by adjusting the molecular weight or terminal groups of polymers.
For example, the first dye-sensitized solar cell using a polymer electrolyte without a solvent was reported by a research group with professor De Paoli of Brazil in 2001, and a polymer electrolyte composed of poly(epichlorohydrin-co-ethylene oxide)/NaI/I2 was prepared to show an efficiency of 1.6% at 100 mW/cm2. Then, a group of researchers called Flaras from Greece mixed a PEO electrolyte of high crystallinity with nanoparticles of titanium oxide and presented a result of PEO with reduced crystallinity in 2002, and the group of Professor Flavia Nogueira prepared titanium oxide into a form of a nano tube using the same poly(epichlorohydrin-co-ethylene oxide) as the group of Professor De Paoli to lower crystallinity thereof and reported the result showing a solar conversion efficiency of up to 3.5%.
However, conventional research studies as described above have little significance in the results of research itself and there has not been any development for commercially available products. Thus, in order to achieve early commercialization of dye-sensitized solar cells by improving performance and durability thereof, there is an urgent need for development of novel polymer electrolytes.
To this end, the present inventors have achieved devising a novel polymer electrolyte and an assembly process thereof that overcomes the limitations of conventional polymer electrolytes based on PEO. Considering the prior art of the industry that though development of new transparent electrodes, technology for new semiconductor materials and for manufacturing the same, technology of dye for absorbing a wide range of wavelength, development of new materials for a counter electrode and techniques for manufacturing the same, and the like have already reached a level of commercialization, there is still the same limitation of using liquid electrolytes, polymer electrolytes in accordance with the present disclosure will have great particular effects in the art to which the present disclosure pertains.